I have been working on a custom power steering reservoir made from three inch aluminum tubing. For the inside of the end caps of the tube I needed to machine a concave or a dish shape. This was especially important for the bottom end cap so that a swirl effect could be created in the fluid flow back to the power steering pump. The fabrication of the parts I made for the power steering system will be an entirely different post at later date but does lead into this post. A few years ago when I purchased the machine shop equipment from an elderly hobbyist, many bits of tooling and attachments where part of the deal. The metal lathe came with a considerable amount of accessories, even an over the top ball cutting attachment.
Of course this ball cutting attachment would not cut a concave or dish shape into a piece of three inch round aluminum stock or cut larger cone or bullet shapes. So I embarked on a new quest. Designing a cone / bullet and concave attachment was an interesting challenge. To design an attachment to machine the concave shape into the round three inch diameter stock wouldn’t be too hard to do. Making the attachment to also function as a cone / bullet cutter, especially for round material up to two inches in diameter would be a bit more difficult. There where all kinds of designs on the internet that would do the job on much smaller diameter stock, but none could be found for what I needed. To start the process off, I thought it best to start with the design of the cone / bullet cutting attachment. On a piece of gridded paper, I drew a length of 1” diameter stock and then using a compass started to play around with the position of the compasses center point and radius. I quickly found that the cutter would need to have a pivot or turning radius of 2.5” for the desired cone shape. When I repeated the process for 2” diameter stock, I determined that the cutter pivot point or radius would need to be 4.5”.
Now armed with a bit of information I took my compound feed off the metal lathe, mounted some 1” diameter stock in the chuck, and started making some measurements on the top of my cross feed. I repeated all of this again for some 2” diameter stock in the lathe chuck. Having all of the information needed for my design, I started looking at what steel I had to make this new cone / bullet cutting accessory for the metal lathe. As it turned out, all I needed to purchase for this project where a few specialty bolts. Fortunately, having a milling machine, simplified the fabrication process and I didn’t need to MacGyver anything. As it turned out, this attachment cuts perfect cone / bullets shapes.
I did attempt to use it for the machining of a 3” x 0.75” concave (dish) in some aluminum. This did not work out so well. There is a very slight amount of play in the pivot point and having the cutting tool 7.5” from the pivot point resulted in an uncontrolled cut. I believe this problem can be rectified by supporting the swing arm of the cone / bullet cutting attachment. That will have to wait for now. In the end, I set up the compound feed to machine a funnel shape into the end caps of the power steering reservoir.
Below is a short video of the cone / bullet cutter in action on my metal lathe cutting a cone shape onto some 1” diameter aluminum stock.